Hello,  I am making my first attempt to work with a non-linear model in R, and am hoping someone can spot the error I've made.  The scenario is to model the rate of gas escaping from a container with 1 vs. 2 vents.   10 container samples were pumped up to 2 psi (relative pressure), then allowed to vent for 15 minutes.  Pressure readings were taken every minute.  The research question is the significance of the Vent factor (1 vs. 2),  with Samples as the random factor and Pressure decaying exponentially.  Readings at time zero were removed, as they were 2 psi.  Any direction you can provide is appreciated, regards, Paul
 
> str(Press.dat)
'data.frame':   125 obs. of  4 variables:
 $ Time     : num  1 2 3 4 5 6 7 8 9 10 ...
 $ Pressure : num  1.8 1.6 1.5 1.4 1.3 1.2 1.1 1 0.9 0.8 ...
 $ Vents    : num  1 1 1 1 1 1 1 1 1 1 ...
 $ Sample   : num  1 1 1 1 1 1 1 1 1 1 ...
 

 
> Press.fit1 <-
+ nlme(model = Pressure ~ Peak * (1 - exp(-Rate*Time)),
+ fixed = Peak + Rate ~ Vents * Time, random = Peak + Rate ~ 1 | Sample,
+ data = Press.dat,
+ start = c(Peak=2, Rate=1.3))
Error in nlme.formula(model = Pressure ~ Peak * (1 - exp(-Rate * Time)),  : 
  starting values for the fixed component are not the correct length
 

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